1. Field of the Invention
The present invention relates to a vehicle having an apparatus mounted thereon for reducing aerodynamic drag and, more particularly, to a guide vane apparatus with an airfoil cross-section mounted to the aft portion of a tractor trailer.
2. Description of the Related Art
Reducing the fuel consumption of passenger vehicles is an urgent world issue that is currently being addressed by scientists and engineers. In addition to passenger cars on American roadways, the US Department of Transportation (2006) reported that there were 2,010,335 combination tractor trailers registered in service in 2005. The average number of miles each vehicle traveled per year was approximately 72,325 miles and over 24.7 billion gallons of fuel was consumed.
At highway speeds, a modern tractor trailer uses 65% of the total engine power to simply overcome drag. A tractor trailer's geometric features contribuuteto the overall drag of the vehicle. At this kind of average road mileage, a 5 percent reduction in fuel consumption would result in billions of dollars of savings for the transportation business, and have an undeniably positive environmental impact.
ATDynamics is a California-based company specializing in tractor trailer aerodynamic solutions. According to the firm, a design with the ability to reduce drag by 12-20% results in a 5.1% fuel efficiency gain at 62 mph. This means that at diesel prices of $3 per gallon, the long-haul trucking industry will realize savings of $1,000-$5,000 per trailer per year. This figure is based on a baseline long-haul trailer traveling 40,000 to 200,000 miles per year at 62 miles per hour.
As fuel costs continue to increase, the need for improved aerodynamic strategies cannot be overemphasized. It has been noted that that the benefits of drag reduction are threefold: reduced fuel consumption, increased acceleration, and increased top speed.
Today there are numerous start-up companies in the United States dedicated to delivering fuel efficient technologies to the trucking industries. Within the most competitive of these companies, in addition to cab fairings, aerodynamic side skirts and wheel covers, there has arisen a strong push towards marketing a device aimed to reduce aft drag in tractor trailers. Solus Inc. is one company that focuses on consultation, research and product development for aerodynamic devices for ground transportation industry. On their website, they detail six different flow modification devices, including side skirts, cross flow vortex strips, vortex strakes and two different aft cavity devices. The firm claims 15% in fuel savings with the use of all their devices together.
Another leading company in the global freight fuel efficiency industry is the California-based firm, ATDynamics. This company got its start in 2006, and is currently using an angled planar cavity device. ATD now markets the design as its TrailerTail®, claiming fuel savings of 5 to 6% for tractor trailers traveling at highway speeds. Engineers at ATDynamics have revised the base design so that it now is easily collapsible to still allow full usage of the doors. According to the company's website, the device can be retrofitted to any swing door trailer, and in early 2009, another design will be released for roll-door trailers.
The reduction in drag associated with tractor-trailers has been the subject of numerous investigations throughout years. Initially, frontal drag was only considered, from the standpoint of changing the shape of the cab. In the early 1970's NASA carried out an extensive investigation into the concept of drag in tractor trailers. In their experiments, researchers modified the flow over the front of the truck. They concluded that with the aid of commercial aerodynamic devices, the fore drag could be reduced by up to 24 percent. This was associated with a reduction of the coefficient of drag, CD, from 1.17 to 0.89. Many more studies throughout the years were carried out to focus on forebody modifications to reduce drag. At that time, the largest drag reductions were available by changing the forebody flow field through the addition of fairings and other flow modifiers.
Research into aft related drag closely followed all of the forebody drag reduction studies. One of the most heavily cited research studies about add-on devices to reduce the aft drag was carried out by Mason and Beebe (1978), in an investigation into flow field characteristics of trucks and buses sponsored by General Motors (see Mason, Jr., W. T., and P. S. Beebe. “The Drag Related Flow Field Characteristics of Trucks and Buses.” Aerodynamic Drag Mechanisms of Bluff Bodies and Road Vehicles. General Motors Research Laboratories, 1978. pp. 45-93). In their work, they investigated the major drag producing regions and detailed characteristics of each of them. Their research is one of the most complete and highly detailed studies of tractor trailer drag. Most notably, Mason and Beebe investigated the potential of increasing base pressure through aft-mounted devices. Many different aft device configurations were studied, including horizontal and vertical splitter plates, non-ventilated cavities, and even guide vanes. All drag reduction concepts were aimed at lowering the base pressure of the trailer, thus reducing the impact of turbulent air shed off the rear of the trailers and the associated drag. Mason and Beebe maintained that an effective design must serve two important purposes: the pressure gradient must be minimized and the large streamline separation associated with moving bluff bodies must be prevented.
At Clarkson University, there have been multiple undergraduate and graduate students who have researched aft reduction devices. The first notable study was done by J. D. Coon (2001), who investigated the effects of a wide variety of unventilated cavity devices on drag. Coon conducted a full experimental and numerical study to examine both the effectiveness and the feasibility of drag reduction using these devices on the aft face of tractor-trailers. By varying the geometric parameters of plate-cavity designs, like plate length and plate inset, an optimum range was found. The performance of the proposed drag reduction devices was evaluated using a 1:15.25 scale model of a Peterbilt 379 tractor and 48 foot trailer. Coon also used a numerical simulation to look at the flow field, and this indicated that the simulation supported that the designs effectively lowered the base pressure of the model. Additionally, he constructed a full-scale prototype of his best performing device and performed several on-road tests. The results of his scale model drag increments obtained in the Clarkson University subsonic wind tunnel were promising, and indicated a drag reduction of up to 9% of the isolated trailer drag.
Dave Maragno (2003) continued to investigate geometric manipulation of the rear of tractor trailers, attempting to narrow the wake region and reduce drag. Maragno specifically focused his research on discovering what the optimal geometries and configurations of planar cavity drag reduction devices. Plate-cavity devices were examined, due to their success in Coon's research, as well as the practicality and ease of use in industrial and commercial settings. In his work, Maragno recognized the success of Coon's planar cavities, but cited explicitly how designs like this become less effective with increasing yaw angles. He investigated angling the planar sides inward to reduce drag more effectively over a larger range of yaw angles. Additionally, Maragno sought to determine which angled plate geometry produces the largest reduction in drag for typical highway conditions. Maragno took into account dozens of geometries and combinations of factors, including the inset distance, angular insets and deflections, side length and top and bottom plate removals, even the addition of splitters. A total of twenty-six designs are fully detailed and assessed for a range yaw angles in his report. The results indicated that angled plates are more effective at reducing drag than previous designs. At zero degrees yaw, Maragno obtained results with maximum ΔCD values of around 0.12. This value was nearly double the ΔCD value of 0.07 that Coon had observed at zero degrees yaw in his best performing device, the 4 foot inset plate-cavity device.
In 2007, Kevin Grover evaluated and optimized these sealed aft cavities for use on full size Class 8 tractor trailers. Through full scale testing, he observed drag reduction results similar to Maragno. Grover maintained that the sealed aft cavity geometry provided the best performance, with a 15 degree inward angular deflection on the side and top panels and a 7 degree inward angular deflection on the bottom panel. The scope of his research mostly focused on the fuel savings numbers associated with the previously mentioned drag reduction devices. His results indicated an optimal savings of 0.6 mpg for a four-sided planar configuration. Based on a nominal fuel economy for a tractor trailer of approximately 7 miles per gallon, this represents an 8.5% savings.
Guide vanes have been studied as an alternative to reducing drag in bluff bodies. The oldest published research found regarding vaned devices aimed to reduce aft drag is referenced in Mason and Beebe's draft reduction studies. This is the wind tunnel investigation of Frey (1933). His research was one of the first studies into reduction of pressure drag by a means of guide vanes. Frey used a simple bluff body which had a span of 500 mm and was tested between non-metric end plates, 1.3 m long×1.0 m wide. He treated this body simply as a two dimensional shape. The effects of adding two staggered guide vanes on the body's trailing corners were investigated. These guide vanes were found to have a dramatic difference on the body's CD. By adding the vanes, a 50% reduction in the body's CD was reported.
In a more recent investigation, as a part of a larger drag reduction study, Mason and Beebe tested the drag reduction characteristics of guide vanes applied to a tractor trailer. Using Frey's results as a motivation, Mason and Beebe also explored the practical potential of using guide vanes. Mason stated that the hope for the vaned device was that the vanes would direct air flow inward, towards the low pressure area behind the trailer, thus significantly reducing the trailer's wake size and drag.
In their results, however, Mason and Beebe actually found that the guide vanes had an adverse effect on drag: creating more, rather than reducing it. They suggested that the dramatic results of Frey do not appear transferable to three-dimensional bodies in ground proximity. They then cited similar results from two other studies: Sherwood (1953) and Kirsh, Garg & Bettes (1973). In their findings, the only device that they found to reduce drag was the non-ventilated cavity design yielding a drag reduction of about 5%. Because of the failure to obtain positive drag-reducing results with the guide vanes, the option has always been mostly ignored as a viable option.
A recent paper published by the Japanese Society of Mechanical Engineers was found, it reopened the possibility of using a guide vaned device to lower aft drag in tractor trailers. In 1986, Kato, Fujimoto and Watanabe three Japanese mechanical engineers, published a paper entitled, “Form Drag Reduction of a Bluff-Based Body with the Aid of Thin Circular-Arc Vanes”. In their investigation, Kato et al attempted to control the wake and reduce drag with the aid of thin circular-arc guide vanes, installed at the base corners. Findings indicated a drag coefficient decrease of about 25%, a decrease even larger than those obtained by using non-ventilated cavity devices. In addition to these positive results, the paper contained an optimum configuration of vanes as a ratio between the bluff body width, its boundary layer thickness and its displacement thickness.
Because Kato's guide vane investigation produced such promising results, there arose reason to believe that these vanes could reduce drag just as dramatically for a tractor trailer bluff body model. Ultimately, the Kato study served as the largest motivation and guideline for the arc shaped guide vanes tested for this investigation. The study's optimum geometries worked well for Kato's bluff body, but dimensions had to be found for a tractor trailer case. Kato's dimensions could've been simply scaled to the Clarkson wind tunnel model, but most likely would not be optimized for maximum drag reduction. Due to the numerous geometric variable dimensions involved with the guide vane placement, a computational fluid dynamic study was conducted first. The CFD study, outlined in the next chapter, was directed toward finding the best geometric placement of guide vanes.
Description of the Related Art Section Disclaimer: To the extent that specific patents/publications/products are discussed above in this Description of the Related Art Section or elsewhere in this Application, these discussions should not be taken as an admission that the discussed patents/publications are prior art for patent law purposes. For example, some or all of the discussed patents/publications/products may not be sufficiently early in time, may not reflect subject matter developed early enough in time and/or may not be sufficiently enabling so as to amount to prior art for patent law purposes. To the extent that specific patents/publications are discussed above in this Description of the Related Art Section and/or throughout the application, they are all hereby incorporated by reference into this document in their respective entirety(ies).